The animal’s level of physiological and behavioural activation at a given moment.

Arousal exists on a continuum from very low (sleepy, disengaged) through optimal (alert, engaged, ready to learn) to very high (over-stimulated, reactive, in flight-or-fight). The continuum is observable across mammals and birds in broadly similar ways, with similar physiological markers (heart rate, respiratory rate, muscle tension, postural changes) and similar behavioural correlates.

Arousal level has a powerful effect on learning, with the relationship typically following an inverted-U curve: too little arousal and the animal cannot engage with the task; too much and the animal cannot learn either. The optimal zone for learning is somewhere in the middle, and the exact position of the optimum varies with the complexity of the task being learned. Simpler tasks tolerate higher arousal; more complex tasks require lower arousal.

Recognising arousal level and managing it is one of the core skills in any training discipline. A horse who is under-aroused is sluggish and inattentive; the same horse at optimal arousal is responsive and learning quickly; the same horse over-aroused is reactive and unable to learn. The same principles apply to dogs in agility training, parrots in trick training, and zoo animals in husbandry training.

Arousal is closely related to but distinct from emotional valence. An animal can be highly aroused in a positive direction (excited, eager) or in a negative direction (anxious, fearful), and the welfare implications of these two states are very different. The combination of arousal and valence forms the two-dimensional framework that contemporary affective science uses to describe affective state across species.

Practical training across species attends to arousal continuously. Skilled trainers can recognise small shifts in arousal in real time and adjust the session accordingly, ending or stepping down work before the animal crosses the threshold where learning becomes impossible.